It is generally well known that the compressive load-carrying capacity of a piece of resilient material may be increased many times over by subdividing the resilient material into a plurality of strata which are perpendicular to the compressive load and separating these strata with intervening strata of non-extensible material. At the same time, however, it has been found that the ability of the resilient material to yield in a direction parallel to the strata remains substantially unaffected.
This concept has been employed in a wide variety of bearing configurations. See, for example, the following U.S. Pat. Nos. and the prior art cited therein: Finney, 4,105,266; Finney, 4,040,690; Johnson, 3,807,896; Peterson, 3,792,711; Dolling, 3,941,433; Schmidt, 3,679,197; Lee et al, 3,429,622; Boggs, 3,377,110; Orain, 2,995,907; Krotz, 3,179,400; Hinks, 2,900,182; and Woldhaber, 2,752,766.
One important elastomeric bearing configuration is the cylindrical bearing. This bearing utilizes a design whereby alternating strata of a resilient material and strata of a non-extensible material are formed as surfaces of rotation about a common central axis, with successive strata being disposed at successively increasing radii from the axis. Such bearings have proven quite useful in applications which require a bearing for carrying relatively large compressive loads in a direction perpendicular to the axis while accommodating some thrust and torsional loads acting on or about the axis.
Experience with such cylindrical bearings has shown that where each resilient stratum in the laminated bearing is formed of equal length, thickness and modulus of elasticity, prolonged use of the bearing in accommodating cyclic torsional movement generally results in fatigue failure in the innermost resilient stratum before similar fatigue failure occurs in the other resilient strata. This preferential fatigue failure is caused by the greater torsional shear strains established in the innermost resilient stratum during torsional movement. As a result, the fatigue life of such a cylindrical bearing is typically determined by the torsional shear strains established in the innermost resilient stratum during bearing use.
This may be better understood by considering that, as is well known in the art (see U.S. Pat. No. 3,679,197), the torsional shear strain carried by any given resilient stratum in an elastomeric bearing in a given torsional load application is an inverse function of the product of the stratum's effective surface area (A) carrying the torsional load applied perpendicular to the surface area, the average radius (R) from the common center axis to that surface area and the shear modulus of elasticity (G) of the resilient material. More specifically, EQU K=RAG (1)
where K=a constant established for the inner layer and used for calculation of remaining layer shear moduli. Thus it will be seen that in the case of a cylindrical elastomeric bearing which has each resilient stratum formed of the same material (i.e. where each resilient stratum has the same shear modulus of elasticity), the strain induced by cyclic torsional movement will be greatest in the innermost resilient stratum due to its smallest radius R and its smallest surface area A. As a result, the innermost resilient stratum is typically the first to fail from prolonged torsional movement when the laminated bearing has resilient strata of equal lengths, thicknesses and shear moduli of elasticity.
One solution to this failure problem was advanced in U.S. Pat. No. 3,679,197. Specifically, in order to provide torsional shear strains across the inner resilient strata which are equal to or approaching the strains across the outer strata, the patentee Schmidt suggests varying the elastomer stock used in each stratum so as to provide each stratum with a different shear modulus of elasticity, in order that the strain across each resilient stratum may be equalized under a given torsional load. In this connection it should be noted that for a laminated bearing of cylindrical cross-section and length L, the ratio of the strain across a stratum i for a given torsional load to the strain across a stratum j for the same torsional load is made equal to one, so that the strains in each stratum are made equal to one another. Putting this in terms of Eg. (1) above, we have ##EQU1## Thus, the strain across any strata i and j will be made equal if the shear modulus of elasticity of the strata varies inversely as the square of the mean radius of the respective area. The patentee Schmidt also states that it is advantageous to progressively increase the thickness of the resilient strata as the radius increases. By progressively increasing the thickness of the strata with increasing radius, Schmidt suggests that more resilient material can be made available within each resilient stratum to help distribute torsion-induced strain while still keeping the compressive stresses within allowable limits. Thus, Schmidt concludes that by both progressively increasing the thickness and progressively decreasing the shear modulus of elasticity of each stratum with increasing radius, optimum bearing design can be achieved.
However, the approach advocated in U.S. Pat. No. 3,679,197 leads to two principle sets of problems, one associated with changing the shear modulus of elasticity for each of the resilient strata and one associated with increasing the thickness of each stratum with increasing radius. Varying the shear modulus of elasticity for each resilient stratum tends to lead to the following problems, among others. First, the patentee's method of achieving a different shear modulus of elasticity for each resilient stratum is expensive in that it requires that each stratum be made of a different elastomeric material. Thus, an elastomeric bearing consisting of fifteen resilient strata would require fifteen different elastomer materials. While the production of these different elastomer materials may be achieved by subdividing a basic elastomer feedstock into fifteen different lots and modifying each lot with a different amount or type of additive, the fact remains that it is costly, time-consuming and inconvenient to provide a different material for each resilient stratum. Second, use of a relatively large number of elastomer materials as suggested by Schmidt is also disadvantageous where the bearings are to be used at relatively low temperature, e.g. -45.degree. to 0.degree. F. This is due to the fact that elastomer stocks tend to behave differently as the temperature is lowered. Thus, in a bearing made according to Schmidt only some of the elastomeric strata may work effectively while the bearing is cold, thereby inhibiting proper bearing performance and accelerating bearing deterioration due to the uneven strains created in the various resilient strata. Third, since each different elastomeric material tends to exhibit unique changes in its shear modulus of elasticity over a range of torsional shear strain magnitudes, formation of each resilient stratum from a single, unique elastomer material tends to provide a bearing which will equalize the strains within each resilient stratum only for a small range of strain magnitudes. However, if the strain magnitudes vary substantially, the shear moduli of each elastomer also will tend to vary substantially and the different strata will no longer experience substantially equal torsional shear strains. Thus, as the bearing of Schmidt is subjected to a wide range of torsional loads, the torsional shear strains in each resilient stratum will tend to differ from the strains created in other resilient strata, so that preferential fatigue will tend to take place once again.
Regarding the second set of problems associated with the approach advocated by Schmidt, increasing the thickness of each resilient stratum as the radius is increased will lead to a reduced number of laminae within the bearing if the overall bearing diameter is kept constant. This may be advantageous in certain applications. Nevertheless the reduction in the number of laminae tends to significantly reduce the compressive loads which can be accommodated perpendicular to the laminae.